Designers of fuel injection systems for internal combustion engines are continuously seeking ways to achieve maximum performance capability while minimizing manufacturing, repair and replacement costs. These objectives are particularly difficult to achieve in the design of fuel injection systems for internal combustion engines of the compression ignition (diesel) type. For example, efficient combustion and low pollution operation of diesel engines requires extremely accurate control over the quantity and timing of fuel injection at very high pressure, i.e. 15,000-20,000 psi and higher. Systems adequate to achieve these objectives are typically complicated and require extremely close manufacturing tolerances. Obviously, these design complications and requirements translate into very high manufacturing and replacement costs.
The assignee of this application, Cummins Engine Company, Inc., has pioneered in the development of a relatively simple fuel injection system for compression ignition engines that optimizes desirable performance objectives but avoids the high costs associated with more complicated systems. This system is known as a pressure/time unit injector system and is disclosed in U.S. Pat. Nos. 3,351,288 and 3,544,008. Essentially the system includes a separate cam operated unit injector for each engine cylinder and a single supply line (common rail) for supplying fuel to all of the unit injectors. Because fuel is metered into each injector through a separate feed orifice, the time during which each feed orifice is open and the pressure within the common rail can be relied upon to control the quantity of fuel metered for injection during each injection cycle. Of particular importance in achieving reduced cost in the Cummins system is the absence of a pressure operated tip valve to form a "closed nozzle" injector. Prior art injector designs, such as illustrated in U.S. Pat. No. 4,092,964, often require a closed nozzle for accurate metering and, thus, the open nozzle Cummins design enjoys a cost advantage because no pressure operated tip valve is required.
One of the problems associated with ignition compression engines equipped with the Cummins pressure/time, open nozzle injection system has been the tendency to resist start-up shortly after being shut down, for example three to twenty minutes following shut down. This characteristic is known as the hot start problem. The severity of the problem is dependent primarily on starting system capability, engine temperature, type of fuel and compression ratio. An associated problem can be excessive smoke and noise even if start up is successfully achieved.
The severity of the hot start problem can range from the engine not cranking through the first compression stroke until the engine has cooled for several minutes, to the engine cranking normally and starting after hesitating slightly on the first compression stroke. Many vehicle operators have a tendency to let off the starter switch when the engine first hesitates and in most cases, when the starter switch is "bumped" the second time, the engine will crank through and start. However, certain operators have experienced significant hot start problems with vehicles equipped with Cummins engines. These operators are typically those who use their vehicles for short pickup and delivery applications with frequent shut downs and startups. A higher incidence of hot start problems occur in colder climates where winter fuel blends of No. 1 and No. 2 diesel fuel are used.
Fuel injectors having closed nozzle tip valves have inherently greater ability to control fuel leakage into the combustion chambers of the engine upon shut down. Such leakage is known to be disadvantageous in certain types of non-Cummins type fuel injection systems. For example, the patent to Bostick et al. (U.S. Pat. No. 4,782,808) discloses a fuel injection system employing solenoid controlled, closed nozzle injectors wherein pressure is relieved upon engine shut down in the fuel supply line leading to the injectors. This pressure relief is designed to prevent fuel leakage through the injectors and into the cylinders, col. 3, lines 57-58. Additionally, this reference teaches that the pressure in the fuel supply line can be decreased after engine shut down by expanding the volume of the fuel supply line by using, for example, a bellows configuration, col. 5, lines 14-16. The purpose of the Bostick et al. system is disclosed to be the reduction in the tendency for carbon and varnish to form in the injectors due to heat build up immediately after engine shut down.
The type of fuel injection system disclosed in the Bostick et al. patent is typically used on gasoline, spark ignition engines which are typified by far lower injection pressures. This lower pressure allows the use of only a single fuel pump for creating the requisite injection pressure for all of the engine cylinders. In compression ignition engines the need for much higher injection pressures necessitates the use of individual cam operated unit injectors positioned adjacent each engine cylinder to avoid the negative effects of pressure waves that would otherwise arise if fuel were supplied at the requisite injection pressure through relatively long conduits.
The Bostick et al. type injectors also employ a solenoid actuated tip valve to control injection timing and quantity. Clearly, injectors of this type are quite different in structure and function from injectors of the type disclosed in the Cummins '288 and '008 patents.
The patent to Knapp et al. (U.S. Pat. No. 4,227,501) discloses a system to allow fuel in the injector fuel supply line to return to the fuel tank when the engine is shut off, thereby preventing evaporation of the fuel in the supply line, which can lead to starting difficulties (vapor lock). Again this patent shows a system suitable for injection of gasoline and fails even to disclose injection directly into a combustion chamber but shows instead injection into the intake passage upstream of the intake valve.
Other types of vapor lock prevention systems have been disclosed including a system (Japanese Patent Document 57-200663A to Yamazaki) which uses a solenoid valve between the fuel supply line and a return line wherein the valve is actuated under certain conditions upon engine shutdown. Again, this patent fails specifically to suggest application of this concept to cam actuated unit injectors for compression ignition type fuel injector systems.
Other examples of systems for removing fuel from injector supply lines are disclosed in the patents to Ulrich (U.S. Pat. No. 4,257,375), Gmelin et al. (U.S. Pat. No. 4,383,513) and Maisch et al. (U.S. Pat. No. 4,530,329).